Microwave hybrid junctions



Feb. 28, 1961 E SALZBERG 2,973,486

MICROWAVE HYBRID JUNCTIONS Filed 001;. 25, 1958 FIG. 3

JNVE TOR. EDWARD SALZBERG BY W 0 ATTORN YS hi it 2,973,486 MICROWAVE HYBRID JUNCTIONS Edward Salzberg, Framingham, Mass, assignor to Microwave Development Laboratories, Inc., Wellesiey, Mass, a corporation of Massachusetts Filed Oct. 23, 1958, Ser. No. 769,155

1 Claim. (Cl. 333-9) called the H waveguide arm, having its axis and wide wall perpendicular to the axes and wide walls, respectively, of the other three. Energy may be transferred from the first and second arms to either or both of the E and H arms and vice-versa. However, the first and E arms are electrically isolated from the second and H arms, respectively.

For a description of the mode of operation of such junctions, reference is made to Patent No. 2,764,740 and articles by W. K. Kahn entitled E-Plane Forked Hybrid-T Junction at page 52 of the IRE transactions-Microwave Theory and Techniques for December 1955 and Patn'ca A. Loth entitled Recent Advances in Waveguide Hybrid Junctions at page 268 of the same publication for October 1956. The former article discloses a structure for achieving a low VSWR in the H arm over a relatively wide bandwidth. However, some applications require a still lower VSWR over a wider bandwidth.

Accordingly, the present invention contemplates and has as a primary object the provision of a folded waveguide T junction which provides a good impedance match to both the E and H waveguide arms over an exceptionally wide frequency band while minimizing the reduction in the power handling capabilities of either.

It is another object of the invetnion to achieve the preceding object by means of a relatively simple structure in which the number and size of matching elements are minimized.

It is a further object of the invention to achieve the foregoing objects by meanswhich permits the E and H arms to be optimally matched by respective matching elements which have virtually no effect on the impedance match of the other arm.

According to the invention, a matching tab extends from the common wide wall adjacent to the rectangular opening where the H arm is joined to improve the match of the latter to such an extent that asimple inductive iris spaced approximately a half guide Wavelength away in the H arm reduces the VSWRL therein to nearly unity over a relatively wide frequencyaband. The E arm is coupled to thejunction by an impedance matching waveguide section dimensioned to providezthe desired im .pedance transformation. A simple inductive iris in the E arm immediately adjacent to the impedance matching seciton reduces the VSWR therein to just above one over an exceptionally wide frequency band. v

Other features, objects and advantages of the invention F i 2,13,486 Y 1Q Patented Feb.28,1961

will become apparent from the following specification when read in connection with the accompanying drawing in which:

Fig. 1 is a perspective view of a preferred embodiment of the invention with portions cut away to expose additional impedance matching elements which further enhance the performance of the junction;

Fig. 2 is a sectional view through section 2-2 of Fig. 1;

Fig. 3 is a Smith chart plot of the complex VSWR as a function of frequency to show the improvement resulting from employing a matching tab to match the H arm; and

Fig. 4 graphically represents the VSWR as a function of frequency within the E and H waveguides energizing a junction formed according to the invention.

With reference now to the drawing, and more particu larly Figs. 1 and 2 thereof, there is shown a perspective view of the novel junction. Two collinear waveguides 11 and 12 having a common wide wall 13, an E waveguide 14 and an H waveguide 15 are coupled together by the junction. The waveguides 11, 12, 14 and 15 are dimensioned to normally propagate the fundamental or TE mode.

The E waveguide 14 is coupled to the junction by an impedance matching section 16. The section 16 has the same wide dimension as guides 11, 12 and 14 but its narrow dimension is selected so that the wave impedance in the section 16 is approximately equal to the square root of the product of the wave irnpedances in the E arm 14 and in the common intersecting volume 19 at the center frequency of the operating band. The length of section 16 is approximately a quarter of the guide wavelength at center frequency. Its precise length is selected to provide the best impedance match as determined by measuring the VSWR with all of the rectangular waveguides terminated with a matching impedance. An inductive matching element 17 contacts narrow wall 18 of the E waveguide 14 at the junction with section 16. This element further reduces the VSWR.

The H waveguide 15 meets the common volume 19 at a rectangular opening 22. Waveguide 15 is formed with an inductive element 23 contacting narrow wall 24. This element is approximately half the guide waveiength at center frequency from the rectangular opening 22 and further reduces-the VSWR of the H waveguide 15.

The common wide wall 13 is formed with a matching tab 25 extending into the common region 19 immediately adjacent to the rectangular opening 22. Preferabiy, the tab 25 is thin so that it interferes negligibly with the matching of E arm 14. T .e width, length and position of tab 25 is selected to minimize the VSWR of H waveguide 15.

Referring to Fig. 3, there is shown a Smith chart plot of complex normalized impedance as a function of fre-' quency at rectangular opening 22. Curve 31' shows this functional relationship from 8.5 kilomegacycles to 9.6 kilomegacycles without a matching tab 25. When matching tab 25 is added, the functional relationship is rcp resented by curve 32 for the same frequency range. Note that without the tab, the VSWR is nearly two. By adding the tab within the common region 19, the VSWR magnitude is less than 1.1 and may be minimized by locating an inductive iris 17 where arm 14 joins section 16.

Referring to Fig. 4, there is shown a graphical representation of'the VSWR as a function of frequency in the E arm 14 and H arm. 15. Note that the VSWR is 1.1

- orbelowfrom 8000'to 9600 kilomegacycles in botharms. An advantage of the particular structure shown and describedisthat E arm 14 may be accurately matched by adjusting the length of section-16 andv then, adjusting the exact position and dimensions of inductive his 17 without disturbing the impedance match of H arm'15f.

3, Similarly, the exact' dimensions and position of tab 25 may be first determined to minimize the VSWR in H arm 15 followed by exactly locating and dimensioning the iris 24 for minimum VSWRI These determinations may be made experimentally without affecting the impedance match of arm 15.

Referring to Fig. 4, there is shown a graphical representation of the VSWR of the E and H arms 14 and 15 as a function of frequency for the embodiment shown in Fig. 1. Note that the VSWR is less than 1.1 from 8000 to 9600 megacycles. This result was achieved with tab 25 spaced .015 inch from rectangular opening 22.

The improved folded T junction described above provides hybrid operation while closely matching signal and load sources over a relatively wide bandwidth. The structure is compact and easy to fabricate. That the E and H arms may be matched independently by dimensioning and locating only associated matching elements facilitates obtaining an optimum match. The small size of the matching tab and inductive irises minimizes the change in the narrowest dimension across the waveguides and the reduction in power handling capabilities.

It is apparent that those skilled in the art may now make numerous modifications of and departures from the specific embodiment described herein without departing from the inventive concepts. Consequently, the invention is to be construed as limited only by the spirit and scope of the appended claim.

Vhat is claimed is:

An improved folded waveguide T hybrid junction comprising, first and second rectangular waveguide sections having a common wide wall, a third rectangular waveguide section having its wide walls generally parallel to the plane of said common wall, a fourth rectangular waveguide section having its wide walls generally perpendicular to said common wide wall plane, the axis of said fourth waveguide section being normal to the parallel axes of said first, second and third sections, means defining a common region for transferring microwave energy between said sections, said common wide wall being confined wholly between said first and second waveguide sections and terminating immediately adjacent to an outside of said common region, means coupling said third waveguide to said common region, said fourth waveguide section exchanging energy with said common region through a rectangular opening, and a matching tab extending into said common region from a relatively small portion of said common wide wall outside of and adjacent to said rectangular opening for lessening the standing wave ratio in said fourth section without afiecting the impedance match of said third section, said means coupling said third section to said common region being a matching section of rectangular waveguide dimensioned to provide an impedance match between said common region and said third waveguide section, and connected Lherebetween.

References Cited in the file of this patent UNITED STATES PATENTS 2,792,551 Smith May 14, 1957 2,840,787 Adcock June 24, 1958. 2,853,683 Murphy Sept. 23, 1958 

